Rei Nakashima

Shorter intervals between great earthquakes near Sendai: Scour ponds and a sand layer attributable to A.D. 1454 overwash

A sparsely documented tsunami in 1454 may subdivide the recurrence interval between the 869 and 2011 tsunamis near Sendai, as judged from geomorphic, stratigraphic, and archival evidence. Pond-filled breaches cut across beach ridges on century-old topographic maps. The basal pond deposit in one of these breaches postdates 1454. Stratigraphy on Sendai Plain includes a sand sheet that contains marine and brackish diatoms. Radiocarbon ages suggest that the sheet dates to 1406–1615 (2σ), and written records for this interval in Tohoku mention a tsunami in 1454. The inferred inundation extended 1.0–2.5 km inland from an approximate medieval shoreline. Simulated tsunamis that best account for the sand sheet require a thrust earthquake of moment magnitude 8.4 or larger. If the sand sheet represents the 1454 tsunami, the two most recent intervals between great thrust earthquakes in Sendai region spanned 585 and 557 years.

Geographic distribution of the late Cenozoic bivalve Fortipecten in the northwestern Pacific

Abstract The distribution and migration history of the genus Fortipecten, a characteristic late Cenozoic bivalve, is evaluated on the basis of specimens from Hokkaido, northern Japan. Fossil occurrences indicate that Fortipecten species lived in Hokkaido from about 7.0 to 1.2 Ma. The geological occurrences and age of Fortipecten in the northwestern Pacific imply that the biogeographic history of the species was strongly influenced by climatic fluctuations. The latest Miocene migration of its geographic range from central Hokkaido southward to northern Honshu was caused by global cooling. Its early Pliocene expansion northward to Kamchatka resulted from warm, high-stand conditions, and its subsequent range contraction resulted from stepwise cooling in the late Pliocene to early Pleistocene. The species became extinct at about 1.2 Ma as the result of extreme climatic cooling. Four species of Astartidae co-occurred with Fortipecten takahashii from three horizons of Hokkaido. The lowermost horizon (about 6–5 Ma) in the Atsuga Formation is correlated with the Astartidae-bearing horizon (5.5–4.8 Ma) in the Bear Lake Formation in southwestern Alaska, and indicates the timing of the initial opening of Bering Strait.

Ontogenetic changes in shell microstructures in the cold seep-associated bivalve, Conchocele bisecta (Bivalvia : Thyasiridae)

Abstract. We described the shell microstructure of the cold seep-associated bivalve Conchocele bisecta using the acetate peel method and scanning electron microscopy, and revealed complex microstructural changes with ontogeny. The shell of the bivalve has outer, middle, and inner layers that consist of aragonite. The outer layer consists of spherulites and spindles of various sizes and shapes; these microstructures are identified as spherulitic, planar spherulitic, spherulitic prismatic, and spindle-like structures. The middle layer is characterized by cone complex crossed lamellar structure in the outer part and crossed lamellar structure in the inner part. The inner layer is composed of cone complex crossed lamellar, fine complex crossed lamellar, and irregular prismatic structures. On the basis of the observations from the umbo to the ventral margin of each shell layer, we recognized two growth stages that are divided by microstructural distributions in the outer and inner layers and the positions of disturbance rings. The early growth stage is characterized by spherulitic structure in the outer layer, cone complex crossed lamellar structure in the inner layer, and scarce disturbance rings. The late growth stage, on the other hand, has specific microstructures (planar spherulitic, spherulitic prismatic, and spindle-like structures) that lie as sheeted layers within the spherulitic structure of the outer layer, and the disturbance rings are situated close to specific microstructures in the outer layer. In addition, alternations between areas that are cone complex crossed lamellar, thin-layered irregular prismatic, and fine complex crossed lamellar structures are observed in the inner layer. The characteristics of both growth stages indicate that shell growth rate is more variable in the late stage than in the early stage, and that growth rate decreases from the early to late stage, possibly caused by a physiological change. This study suggests that it is important to examine shell microstructural distribution to reveal shell growth and life history.

Shell Microstructures of Five Recent Solemyids from Japan (Mollusca: Bivalvia)

Abstract. Shell microstructures of five solemyid species from Japan were characterized by scanning electron microscopy. All five species examined had outer and inner shell layers, and were categorized into four groups according to the shell microstructure composition. Group 1 consist of Solemya (Petrasma) pervernicosa and Solemya (Solemya) tagiri. The outer layer of their shells is characterized by radially elongate simple prismatic structure (RESP) type A, and the inner layer by irregular prismatic structure. Group 2 is represented by Solemya (Solemya) pusilla. The outer layer of its shell has RESP type B structure, and the inner layer structure is homogeneous. Acharax japonica is in Group 3. Its outer shell layer has RESP type C, and the inner layer is characterized by laminar, homogeneous, and irregular complex crossed lamellar structures. Group 4 is represented by Acharax johnsoni. Its outer shell layer has a reticulate structure and the inner layer has a cone complex crossed lamellar structure. Reticulate microstructures have never before been found in shells of any molluscan species. These groupings indicate that these solemyid species have a variety of shell microstructures, but the groups are not consistent with previous solemyid systematics at the generic and subgeneric level. Solemyid shell microstructures may provide crucial signals for their phylogenetic grouping, but their significance is not clear from analyses of morphological characters only. In future studies, the utility of microstructural characteristics should be confirmed by mapping the character states of a larger taxonomic sample onto molecular phylogenetic trees.